1. Field of the Invention
The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to a method of fabricating an image sensor package.
2. Description of the Related Art
Image sensors are well known to those of skill in the art. An image sensor included an active area, which was responsive to electromagnetic radiation. The image sensor was used to fabricate an image sensor assembly.
In one prior art image sensor assembly, the image sensor was located within a housing, which supported a window. Radiation passed through the window and struck the active area of the image sensor, which responded to the radiation.
To form the image sensor assembly, the image sensor was mounted to a printed circuit mother board. After the image sensor was mounted, a housing was mounted around the image sensor and to the print circuit mother board. This housing provided a seal around the image sensor, while at the same time, supported a window above the image sensor.
As the art moves to smaller and lighter weight electronic devices, it becomes increasingly important that the size of the image sensor assembly used within these electronic devices is small. Disadvantageously, the conventional image sensor assembly described above required a housing to support the window and to seal the image sensor. However, this housing was relatively bulky and extended upwards from the printed circuit mother board a significant distance, e.g., 0.100 inches (2.54 mm) to 0.120 inches (3.05 mm) or more. As a result, the image sensor assembly was relatively large.
In the event that moisture was trapped inside of the housing, defective operation or failure of the image sensor assembly was observed. More particularly, the moisture had a tendency to condense within the housing and on the interior surface of the window. Even if the housing later dried out, a stain was left on the window. In either event, electromagnetic radiation passing through the window was distorted or obstructed by either moisture condensation or stain, which resulted in defective operation or failure of the image sensor assembly.
For this reason, an important characteristic was the temperature at which condensation formed within the housing of the image sensor assembly, i.e., the dew point of the image sensor assembly. In particular, it was important to have a low dew point to insure satisfactory performance of the image sensor assembly over a broad range of temperatures.
In accordance with the present invention, an image sensor package includes an image sensor having an active area and bond pads on a front surface of the image sensor. A window of the image sensor package has an interior surface and an exterior surface opposite the interior surface. Electrically conductive interior traces are formed on the interior surface of the window. The window is mounted to the image sensor by flip chip bumps formed between the bond pads of the image sensor and the interior traces on the interior surface of window.
The window includes a central region aligned with the active area of the image sensor. A peripheral region of the window includes the interior traces, electrically conductive exterior traces formed on the exterior surface of the window, and electrically conductive vias electrically connecting the exterior traces to the interior traces. Electrically conductive pads are formed on the exterior traces. Electrically conductive interconnection balls are formed on the pads.
Advantageously, the interior traces, the vias, and the exterior traces are formed within the peripheral region of the window and the central region of the window remains unobstructed. Accordingly, radiation passing through the central region of the window, which strikes the active area of image sensor, is not obstructed or distorted.
The window has an area less than an area of the front surface of the image sensor. Advantageously, this allows the image sensor package to be the size of the image sensor, i.e., the image sensor package is chip size. Further, by avoiding the use of the housing of a prior art image sensor assembly, the image sensor package in accordance with the present invention can also be made relatively thin. Accordingly, the image sensor package is extremely well suited for use with miniature lightweight electronic devices, which require small, thin and lightweight image sensor assemblies.
The window, a bead between the window and the image sensor, and the image sensor define a sealed cavity in which the active area is located. Advantageously, the volume of the cavity is relatively small. By minimizing the volume of the cavity, the amount of any moisture trapped within the cavity is also minimized. This, in turn, essentially eliminates the possibility of moisture condensation on the interior surface of the window or on the active area of the image sensor. As a result, the image sensor package has a very low or nonexistent dew point.
In an alternative embodiment, the cavity is completely filled with a transparent encapsulant. Advantageously, by eliminating the prior art cavity between the active area and the window, the possibility of moisture condensation within the cavity is also eliminated. Accordingly, the image sensor package in accordance with this embodiment has no dew point.
Further, by using a transparent encapsulant having a refractive index similar to the refractive index of the window, the amount of reflected radiation is minimized. This improves the sensitivity of the image sensor package compared to prior art image sensor assemblies.
Also in accordance to the present invention, a method of forming an image sensor package includes forming an electrically conductive exterior trace on an exterior surface of a window. An electrically conductive interior trace is formed on an interior surface of the window, the interior trace being electrically connected to the exterior trace. The interior trace is aligned with a bond pad on a first surface of an image sensor. An electrically conductive bump is formed between the interior trace and the bond pad thus mounting the window to the image sensor.
These and other features and advantages of the present invention will be more readily apparent from the detailed description set forth below taken in conjunction with the accompanying drawings.